Known projectile systems employ guided missiles flying "minimum energy" trajectories. One radar tracks the target. A second radar tracks the missile. On the basis of these two sets of measurements the missile is commanded to intercept the target. If the two radars are not individually aligned exactly to a common coordinate system, the difference in alignment will appear in a projectile miss component at the target. For example, if this "boresighting" procedure ends up with a 2 mil elevation difference between radars, and there are no other system errors, the missile will miss the target by about 10 meters at 5 km. Range "biases" across radars also contribute to the miss vector, if not removed in radar calibration. Boresighting takes time, must be done repeatedly since radar tends to "drift" off calibration, and requires skilled personnel. It is especially difficult in a mobile field operation where the equipnment is jolted and vibrated during cross country moves. These error sources are probably the reason that recent projectile systems do not employ the minimum energy predicted point type of solution.
Several known projectile systems currently fly "line of sight" trajectories to intercept. The missile positions are measured relative to target position by a single radar, hence there is no "boresight error". However, since the "line of sight" to the target is in motion, the missiles must develop a continuous lateral acceleration to stay on the beam. Acceleration requirements can be as high as 10 g (gravities); the development of lift force of this magnitude requires relatively large lift surfaces, and the drag induced by lift consumes propellant and kinetic energy. Hence these missiles tend to be relatively large, costly, and only rocket propelled vehicles have been feasible. By contrast the energy expenditure to fly a "minimum energy path" is quite small.
Many air defense missiles have homing heads which (1) illuminate the target by radar for the missile and home on the reflected radiation, or (2) illuminate the target from the ground by radar, with missile homing on the reflected radiation, or (3) sense the infrared (IR) radiation of the target and home on it. Radar homing heads are expensive; IR homing heads have difficulty in sensing the target in its forward aspect, and are also expensive.
The concept of measuring the miss vectors of unguided projectiles at the target and using this principle to correct the fire control algorithms has been used in antiaircraft gun systems for decades. Automatic operations of this concept with uncontrolled projectiles is employed by known projectile systems.
There are certain disadvantages in these known projectile control systems where although certain ones fly a minimum energy path, each is vulnerable to boresight errors which could be large and unpredictable in a field mobile installation. Others have eliminated boresight errors, but fly a trajectory that requires hgh energy expenditure. Missiles with homing heads using radar target illumination are costly because of the expense of the homing head. Missiles using IR homing heads have difficulty in sensing the target in the forward aspect and are also costly. Use of miss measurements to correct the fire control system for biases has the disadvantage that the gun-fired projectiles cannot be controlled in flight, hence the system is vulnerable to large errors caused by target maneuvers, as well as being severely limited in maximum range.